If you have e-filed, wait until your return is accepted or rejected by the IRS.
If your return is rejected, add your student loan interest (along with anything else that needs to be corrected for e-filing) and then re-submit your return.
If your return has been accepted or you have paper-filed your return, you cannot change the information. Instead, you have to amend a return.
If your original return generates a refund, you should wait until your original refund arrives before you file your amendment Form 1040-X. You can cash your check, but you still should wait until your current tax return is resolved.
For more information on requirements, see https://www.irs.gov/taxtopics/tc456.html
To amend, click here:
https://ttlc.intuit.com/replies/3288565
[Edited 1/28/2017]
